1. Field of the Invention
The invention relates to an apparatus for use in measuring the reflected light intensity of a scene. More particularly the invention relates to an improved apparatus that includes means for attenuating the scene light being measured without causing the apparatus to have unequal sensitivity to light being reflected from different portions of the scene.
2. Description Relative to the Prior Art
It is well known to use a photoelectric cell in an apparatus to measure the reflected light intensity or illuminance of a scene. Such measurement can then be used to determine the correct setting for an exposure-control mechanism in a photographic camera or for other purposes. The photoelectric cell produces an electrical current or varies the resistance in an electrical circuit in approximate proportion to the average intensity of illumination reflected by the scene on a photosensitive surface in the image area (the total area on a surface of the photoelectric cell onto which an image of the scene is focused) of the photoelectric cell. An optical lens is included in the apparatus to direct the image of the scene to be measured onto the image area of the photoelectric cell.
Often a light-attenuating device is included in the apparatus and is positioned between the photoelectric cell and the scene to produce a known set of exposures on the photoelectric cell. One type of light-attenuating device is an optical wedge which in general consists of a strip of material--glass, celluloid or plastic--covered with a pigment or developed silver emulsion layer which is clear at one end and gradually becomes opaque towards the other end. The transition from clear to opaque usually takes place smoothly but may take place in regular steps in which case it is called a step wedge. Such a device is mounted to be moved transversely with respect to the optical path, this lateral movement being parallel to the direction of the density gradient of the device so that the average intensity of the radiation falling on the photoelectric cell image area can be attenuated to a desired degree. Such a device can be used by moving it laterally to keep the radiation intensity within the working range of the photoelectric cell; or it can be used to keep the average intensity of radiation falling on the entire photoelectric cell image area at a constant level. The position of the device thus serves as a means for indicating the proper exposure setting for a photographic camera. It is also known to have the device coupled to the exposure-control mechanism of a camera, so that when the device is moved to keep the average intensity of light impinging on the photoelectric cell image area constant, a proper setting of the exposure-control mechanism occurs automatically.
One problem with such a radiation-attenuating device is that the use of such an optical wedge inherently attenuates light impinging upon one side of the photoelectric cell image area more than it attenuates light impinging upon the other side. The result is a weighted "measurement" of the scene by the photoelectric cell, i.e., the photoelectric cell will give more weight to its measurement of illumination of some portions of the image than to others. In nonhomogeneous scene lighting conditions this weighting (or, in effect, sensitivity distortion) can often result in an incorrect determination of the proper exposure setting for a camera.
The prior art discloses a mechanism intended to solve this problem. U.S. Pat. No. 2,806,405 suggests placing a second optical wedge, fixedly positioned, in the optical path to compensate for the sensitivity-distorting effect of the movable first optical wedge. The density gradient of the second optical wedge has a sense opposite to that of the first to thereby provide a more uniform illumination of the area of the photoelectric cell. A number of disadvantages are associated with such a mechanism. Firstly, the positioning and mounting of another element in the optical path requires more space and increases the likelihood for error in the mounting of all of the other elements in the optical path. Secondly, the presence of another optical wedge in the optical path inherently increases the diffusion of radiation on its way to the photoelectric cell. This means that any given portion of scene light (i.e., unit area) impinging on the light sensitive area of the photocell is less likely to correspond to the actual light intensity of the corresponding portion of the scene. Thirdly, having another optical wedge or gradient in the optical path necessarily decreases the apparatus' overall sensitivity to radiation by constantly keeping a certain percentage of the radiation from reaching the photoelectric cell. The present invention is directed to providing an apparatus that minimizes the problem of sensitivity distortion caused by the light-attenuating device while also minimizing all of the above-noted disadvantages of the solutions suggested in the prior art.